In the following description the molded article to which reference is made will be understood to include shaped bodies generally, and, in particular, hollow bodies such as tubular or pipe-shaped articles. However, the invention is also applicable to the production of other shaped articles such as foils, film, sheets and the like.
In the fabrication of conventional molded or shaped articles, generally aromatic polyesters or copolyesters, especially polyethylene terephthalate and its copolymers are used with small amounts or proportions of, for example, isophthalic acid or cyclohexanedimethanol, polybutyleneterephthalate, polytrimethyleneterephthalate, polyethylenenaphthalate and their copolyesters as starting materials. The starting material after a melt polycondensation with a certain intrinsic viscosity (IV) can be injection molded in an injection molding machine to produce hollow articles. The intrinsic velocity for example in the case of polyethyleneterephthalate and its copolyesters with a low degree of modification, can range between 0.65 and 0.90 dl/g.
That method has been a standard method for producing PET food containers, especially bottles.
In this manufacturing process terephthalic acid or its esters are esterified with ethyleneglycol in an esterification stage comprised of one or more reactors in cascade.
These esters are subjected to polycondensation by an increase in temperature and reduction of pressure in the melt to a PET with an average viscosity and an IV of 0.55 to 0.65 dl/g. The product is cooled and granulated. The polycondensation usually is carried out in at least two stages in a so-called precondensation reactor and a final reactor following the precondensation reactor, the two reactors being referred to generally as condensation reactors. A greater number of condensation reactors may be connected in cascade depending upon the capacity of the apparatus which is desired and the conditions.
The melt obtained from the final reactor is granulated and the PET granulate can be fed to a reactor for solid state polycondensation (SSP) under inert gas at temperatures of 180° to 230° C. to a mean viscosity of 0.75 to 0.85 dl/g. This granulate is as a rule the final product and can be sold as such. Fabricators purchase this granulate and can use the same to produce hollow articles by molding processes and in molding machines. For example, different types of molding units may utilize such granules, including injection molding and blow-molding machines.
Usually preform machines operating in accordance with injection molding principles can produce parisons or blanks, also known as preforms which can then be converted in blow-molding machines, generally by other processors and at other locations, into polyester bottles or like containers. Machines are also provided for the fabrication of films and foil from the polyester granulate.
The solid state polycondensation (SSP) is used mainly for two reasons. Firstly to obtain a sufficient mechanical stability of the finished bottle, the viscosity of the polyester must exceed levels in polyesters used for textile applications. Secondly, the acetaldehyde content in the polymer melt leaving the final reaction must be reduced from about 30 to 70 ppm, to below 2 ppm following the conclusion of the solid phase polycondensation so that the acetaldehyde will not affect the taste of products packaged in the finished PET bottle. The acetaldehyde (AA) results as an unavoidable byproduct in PET production. Above all it is essential for taste reasons to keep the AA content of the PET of the finished bottle as low as is possible. The proportion of the AA in the PET of the bottle can be controllable to a certain degree by the technological conditions of polycondensation and the subsequent solid phase polycondensation. Depending upon the pretreatment of the polymer melt or its thermal history, the conditions in the solid phase polycondensation and the operation of the preform machine, acetaldehyde is formed during the phase of melting of the granulate. During the bottle formation in the blow-molding machine, the acetaldehyde concentration changes only marginally.
In the finished bottle when the latter is filled with soft drinks, the AA value should not exceed 8 ppm and in the case of filling with water, should not exceed 4 ppm.
The SSP requires comparatively expensive apparatus. Before the SSP is carried out the amorphous chips must be crystallized in a comparatively expensive crystal stage to avoid adhesion in the following solid phase polycondensation. In both stages significant amounts of inert gas are required which must be additionally cleaned after use so that it can be recycled in the process.
In general it has been found to be advantageous to produce the preforms on machines specifically selected for this purpose and to which the finished polyester melt can be fed directly. It has not been found to be advantageous to use the costly and time-consuming steps employed in fiber production but rather to ensure that the melt from the polycondensation stage will directly pass to the SSP.
DE 195 03 053 describes a method in which the melt from the polycondensation reactor is treated with inert gas and an AA-reducing low volatility amide compound over the stretch provided with static mixing elements and is subjected to vacuum degassing in the shortest possible time and with the lowest possible shear, can be fed to a molding device for the production of preforms.
In DE 195 05 680 inert gas is fed to the polycondensation melt with an IV=0.5-0.75 dl/g in a postcondensation reactor under vacuum until the product is polycondensed to a viscosity of 0.75-0.95 dl/g and the melt is then directly and immediately fed to an injection molding tool.
EP 0 842 210 describes another possibility for avoiding SSP. There the melt polycondensation is carried out to a viscosity of 0.65-0.85 dl/g, the polyester is cooled and is granulated, remelted and after a formation of greater surface area, is subjected to flushing with a suitable flushing agent from volatile substances like acetaldehyde.
In EP 0 842 211, a method has been proposed in which the polycondensation melt is fed to a degassing extruder with a polymer compression seen, simultaneously with a suitable flushing agent and the thus treated melt is directly fed to a shaping unit.
EP 0 836 548 describes how a polycondensation melt is fed in apparatus terms through a mixing stretch and a distributor into an injection molding unit without providing details of the process.
U.S. Pat. No. 6,099,778 discloses a method in which a polycondensation melt is directly fed to a shaping unit. The method is linked to conditions requiring the catalyst for polycondensation to be free from cobalt, an acetaldehyde reducing compound to be added and the melt to be degassed before it is fed to the shaping device at a pressure from above 25 mm Hg to standard pressure, whereby the degasification unit is comprised, for example, of a degasification extruder or some other appropriate conventional apparatus as the acetaldehyde reducing compounds, polyamide, polyesteramide and polyethyleneisophthalate are mainly mentioned.
WO 98/41381 describes an apparatus and a continuous process for producing shaped polyester articles with low acetaldehyde content from a polycondensation melt without intervening solidification of the polyester. The polycondensation melt is mixed in an extruder under pressure with an inert gas, the melt is degassed under vacuum and reacted in a mixing in the zone with an acetaldehyde reducing compound and then is fed to the injection molding unit. The acetaldehyde reducing agent can in principle be those mentioned in the previous paragraph.
EP 0 968 243 describes a similar process. The polycondensation melt is there fed into a mixing unit which can be comprised of a static mixer, a gear pump or an extruder. A stripping agent like nitrogen or carbon dioxide and an AA reducer like a polyamide or polyesteramide are added. The melt is fed from this mixing unit through one or more nozzles into a rapid evaporator. There it is degassed under a vacuum of 5 to 50 mm Hg and fed to a shaping unit. An AA reducer can be supplied to the latter as well.
In another proposal Feb. 25/26, 2003, inventa-Fischer has suggested a process for making preforms directly from polycondensation melt. For that purpose a high viscosity reactor would be built into the line to supply PET prepolymer and the viscosity would be raised to 0.85 dl/g. Then an AA reducing agent optionally other additives would be fed to the melt and the mixture fed by a mixer into the injection molding machine.
In DE 100 45 719, a method has been described which branches a part of the polycondensation melt from the final reactor and supplies an AA reducer like an amides of a polycarboxylic acid and multivalent amines as well as polyester stabilizers like for example triethylenephosphate to a branched stream. In the same extruder gaseous reaction products are removed by degasification fittings. The partial stream is then rejoined with the main stream. This method has the advantage that the expensive degasification extruder need only be outfitted for a partial stream of the polycondensation melt and therefore can be less expensive. The system nevertheless requires degasification.
As further possible AA reducing agents which can possibly avoid use of SSP, are the compounds proposed in U.S. Pat. No. 6,274,212, namely compounds which have two hydrogen atoms substituted as carbons connected with heteroatoms and which form organic compounds with acetaldehyde with reaction in the polyester and include at least two heteroatoms in an unbridged 5 or 6 member ring. A possible compound of this type is anthroanilamide. This additive can be sprayed as a suspension, for example, on a polyester granulate, for example, a master batch granulate or can be admixed with a melt formed by melting the granulate.
The described methods have the drawback that they require the use of expensive carrier gases for the melt degasification, additional apparatus for degasification and large and expensive equipment and depending upon the point at which the acetaldehyde is removed, the use of scavengers to prevent or reduce new formations of acetaldehyde.